In the semiconductor industry, plasmas are used to deposit materials on and etch materials from work-pieces to form specific electronic components on the work-piece substrate. Plasmas that are used in semiconductor fabrication processes are commonly generated by ionizing a gas in a vacuum chamber. For example, a gas can be introduced into an evacuated plasma chamber where the work-piece is located. A strong electric field can be applied to the plasma chamber. The gas can form a plasma in an excitation region by undergoing an electrical breakdown under the influence of the strong electric field.
The electric field in such chambers is typically either capacitively or inductively coupled to the gas to remove electrons therefrom. In capacitively coupled plasma generators, the electric field can be applied between electrodes on opposing sides of the chamber. In inductively coupled plasma generators, the electric field can be applied to a coil surrounding the chamber.
Vacuum chambers can be problematic and unduly expensive for use in certain applications. Vacuum plasma chambers can be disadvantageous because certain vacuum components are expensive, cumbersome and require high maintenance. Accordingly, plasma generators have been developed which can produce plasma at atmospheric pressure.
Methods of producing plasma at atmospheric pressure include dielectric barrier discharge, arcs and plasma jets. These types of plasma generators tend to produce non-uniform plasma distributions, which are often referred to as filamentary discharges, in the chamber. Such atmospheric pressure plasma generators have a tendency to damage surfaces of work-pieces. For example, work-pieces in presently known atmospheric pressure plasma generators can be subject to arcing and/or levels of directional current which can cause heat damage to their surfaces. Methods for converting the discharge into a broad-area ‘glow’ discharge and thus reducing the resultant heat damage in atmospheric pressure plasma chambers have been attempted by adding helium to the chamber. These methods have been less than optimal because they consume excessive quantities of helium.
Plasma jets are known which use a microwave source to produce a plasma distribution that is very localized. Like arcing, the localized high ion flux that occurs in plasma jet systems can damage a surface such of a work-piece.
In certain plasma generators, the power source that drives the electrodes and/or coil is a high frequency (HF) source in the radio frequency (RF) range or ultra-high frequency (UHF) range, i.e., from about 30 kHz to about 300 GHz. The HF source typically controls the plasma density in the chamber. Certain dual-frequency plasma generators also include a low frequency (LF) source or direct current (DC) source driving one or more electrodes in the plasma chamber. The LF or DC source typically controls the energy of ions that strike a work-piece in the chamber. The use of two power sources makes this type of plasma generator very expensive.